/**
 * Copyright 2010 Neuroph Project http://neuroph.sourceforge.net
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.neuroph.core.transfer;

import java.io.Serializable;

import org.neuroph.util.Properties;

/**
 * <pre>
 * Tanh neuron transfer function.
 *
 * output = ( e^(2*input)-1) / ( e^(2*input)+1 )
 * </pre>
 *
 * @author Zoran Sevarac <sevarac@gmail.com>
 */
public class Tanh extends TransferFunction implements Serializable {

    /**
     * The class fingerprint that is set to indicate serialization
     * compatibility with a previous version of the class.
     */
    private static final long serialVersionUID = 2L;

    /**
     * The slope parametetar of the Tanh function
     */
    private double slope = 2d;

    /**
     * Creates an instance of Tanh neuron transfer function with default
     * slope=1.
     */
    public Tanh() {
    }

    /**
     * Creates an instance of Tanh neuron transfer function with specified
     * value for slope parametar.
     *
     * @param slope the slope parametar for the Tanh function
     */
    public Tanh(double slope) {
        this.slope = slope;
    }

    /**
     * Creates an instance of Tanh neuron transfer function with the
     * specified properties.
     *
     * @param properties properties of the Tanh function
     */
    public Tanh(Properties properties) {
        try {
            this.slope = (Double) properties.getProperty("transferFunction.slope");
        } catch (NullPointerException e) {
            // if properties are not set just leave default values
        } catch (NumberFormatException e) {
            System.err.println("Invalid transfer function properties! Using default values.");
        }
    }

    @Override
    final public double getOutput(double net) {
        // conditional logic helps to avoid NaN
        if (net > 100) {
            return 1.0;
        } else if (net < -100) {
            return -1.0;
        }

        double E_x = Math.exp(this.slope * net);
        this.output = (E_x - 1d) / (E_x + 1d);
//        this.output =  Math.tanh(2.0d/3.0*net) ;
//        this.output = Math.tanh(net);

        return this.output;
    }

    @Override
    final public double getDerivative(double net) {
        return (1d - output * output);
    }

    /**
     * Returns the slope parametar of this function
     *
     * @return slope parametar of this function
     */
    public double getSlope() {
        return this.slope;
    }

    /**
     * Sets the slope parametar for this function
     *
     * @param slope value for the slope parametar
     */
    public void setSlope(double slope) {
        this.slope = slope;
    }
}
